The present disclosure relates to a high thermal conductivity composite structure, and specifically to a composite structure having a connected percolating thermally conducting network structure comprising a thermal interface material (TIM), and methods of manufacturing the same.
Thermal interface material (TIM) is a material used to minimize the contact thermal resistance between surfaces and provides a low resistance path to spread and remove heat. Examples of TIM known in the art employ randomly distributed filler materials dispersed in a matrix. Each of the randomly distributed filler materials has a characteristic dimension (such as a diameter) on the order of several microns or greater.
In such conventional dispersed systems, the filler loading needs to be at least 50˜60% in order to achieve a high thermal conductivity for the composite structure. The associated increase in stiffness, i.e., an increase in the viscosity or compressibility, introduces challenges in manufacturing processes (dispense and handling) and product applications (stresses on fragile components, or accommodating stack-up tolerances). Highly filled conventional TIMs require further process or design improvements to overcome such drawbacks. There is, therefore, a need for thermal interface materials with high thermal conductivity without the drawbacks associated with the high filler loading of conventional high thermal conductivity TIMs.
Typically, the thermal conductivities of matrix materials are much lower than the thermal conductivities of conductive filler materials dispersed therein. A significant level of filler loading, i.e., the volume percentage of the filler material as a fraction of the entire volume of a composite structure including the filler material and the matrix, e.g., 50%-60%, is needed to provide a thermal conductivity that is effective in removing heat. Decreasing the particle size typically results in reduction of the conductivity of the composite structure. While increasing the particle size above 10 microns can increase the conductivity of the composite structure, such large particles and/or increased filler loadings result in dilatant behavior in grease type TIMs and/or an increase in the Bond Line Thickness (BLT) of the composite structure, and therefore, an undesirable increase in thermal resistance, i.e., an undesirable decrease in thermal conductance of the composite structure.
Fast and efficient exchange and storage of thermal energy plays a vital role in a wide range of thermal management applications ranging from buildings and solar power plants to computer chips. Efficient solutions for thermal energy storage and exchange in buildings (one of the largest energy consumers) are needed to enable heat absorption during the times when thermal energy is abundant (and therefore cooling the environment) while releasing the heat during the times when room temperature drops below the setpoint conditions. This approach can lead to smaller temperature variations and reductions in the energy consumption allocated for heating and cooling.
The same general idea leads to better efficiencies in large scale applications such as solar-thermal power plants, and other thermal energy harvesting applications where the power cycle is intermittent, or there is a significant time lapse between the power generation peak and consumer peak use. Phase change materials (PCM) can absorb and release large amounts of thermal energy at relatively constant temperature and pressure. PCMs can be employed in fields such as spacecraft thermal systems, softening of exothermic temperature peaks in chemical reactions, thermal comfort in vehicles, cooling of engines (electric and combustion), medical applications (transport of blood under stable temperature, operating tables, hot-cold therapies), thermal protection of electronic devices (as a passive patch integrated in the appliance), safety (temperature maintenance in rooms with computers or electrical appliances), and thermal storage of solar energy.
One type of phase change mechanisms is the liquid-solid transition. PCMs include three categories of materials. A first category of PCMs include organic PCMs such as paraffin (CnH2n+2) and fatty acids (CH3(CH2)2nCOOH)2. A second category of PCMs include inorganic PCMs, which include salt hydrates (MnH2O). A third category of PCMs include eutectics, which can be a eutectic of at least two organic PCMs, a eutectic of at least two inorganic PCMs, or a eutectic of at least one organic PCM and at least one inorganic PCM.
Commercial paraffin waxes (not pure) are cheap and have moderate thermal storage densities (˜200 kJ/kg or 150 MJ/m3) and a wide range of melting temperatures (20° C.˜70° C.). Such waxes undergo negligible sub-cooling (the temperature below saturation to initiate the solidification process) and are chemically inert and stable. Fatty acids (such as capric, lauric, palmitic and stearic acids) and their binary mixtures melt between 30° C. to 65° C., while their latent heat of transition was observed to vary from 153 to 182 kJ/kg. Hydrated salts are attractive materials for use in thermal energy storage due to higher volumetric storage density (˜350 MJ/m3), and moderate costs compared to paraffin waxes. One of the most important examples of eutectics phase change materials are encapsulated PCMs. Encapsulated PCMs are composed of a protective shell and one or more active materials as the core substance. The protective shell is either natural or synthetic polymer while the active ingredient is mostly a solid.
Thermophysical properties of phase change materials are known in the art. A vital problem of all PCM based thermal energy storage systems is the low thermal conductivity (κ) of the PCM, which restricts the heat transfer rate as well as the fast access to the thermal energy stored away from the heat transfer surfaces. The thermal conductivity of PCM materials is typically much lower than 1 W/mK. In addition, most available commercial PCMs are corrosive to most metals and suffer from decomposition, high changes in volume during phase change, and flammability that can affect phase change properties. These disadvantages keep conventional PCMs from large scale implementations.
At much smaller scales, a similar need for high κ and high heat capacity materials is encountered in the thermal management of electronics and optoelectronic devices. Stable and low operating temperatures are desired for device performance. To improve the heat transfer from the hot side to the heat sink, a critical role is played by thermal interface materials (TIM) used to minimize the contact thermal resistance between surfaces and provide a low resistance path to spread and remove heat. Thermal interface materials known in the art employ microparticles dispersed in a matrix, and include thermal greases, thermal gels, phase change materials, and thermally conductive adhesives based on a matrix and highly conductive fillers such as graphite, carbon nanotubes, silicon carbide, boron nitride, aluminum nitride and aluminum oxide.
The filler loading is high, and is typically greater than 50 volume percent in such TIMs in order to achieve a high composite (effective) thermal conductivity. The associated increase in viscosity or stiffness/compressibility introduces challenges in manufacturing processes (dispense and handling) and product applications (stresses on fragile components or accommodating stackup tolerances). Highly filled conventional TIMs require further process or design improvements to overcome these drawbacks. Thermal greases filled with aluminum, alumina and silver powders have a κ in the range of 2.89 W/mK to 7.5 W/mK. Thermal greases are limited by pump-out resulting from an expansion and contraction at the thermal interfaces. This pumping effect can push the material away from the interface and cause a hot spot. Tin solder joints, consisting of metallic elements, may reach a κ of about 43 W/mK. Poor wetting of the surface, low yield strength and melting point and environmental health concern and package stresses make solder systems an inconvenient material for TIM. Thermal conductivities of 20˜86 W/mK can be achieved with elemental phase change materials such as indium and gallium. These also require complex processing and structure modifications to address adhesion, containment and package stress considerations. Usage of liquid metals between the mating surfaces may also cause corrosion damage. Thermal conductivities of 80 W/mK were reported for a thermally conductive silver paste under the high pressure, high temperature (˜300° C.) sintering process. However the high sintering temperature (˜300° C.) of the commercial silver pastes makes such pastes an impractical thermal interface material. Therefore there is a need for thermal interface materials with high κ without the drawbacks associated with the high filler loading of conventional high thermal conductivity TIMs. For systems where high power levels are switched to help with overall efficiency, TIMs will need to have high effective κ and high effective heat capacities to minimize thermal transients and their mechanical consequences.